Method for controlling automatic transmission of a vehicle on a downhill slope

ABSTRACT

The invention concerns a method for controlling automatic transmission ( 3 ) of a vehicle comprising an engine ( 2 ) driving the transmission ( 3 ) which consists in: detecting a downhill situation of the vehicle, and selecting a transmission ratio so that the engine absorbs energy, storing a longitudinal speed of downhill start (Vmin) when the vehicle starts going downhill, and while the vehicle is running downhill, comparing the current speed (V) of the vehicle with the downhill start speed (Vmin) such that the current speed (V) exceeds the downhill start speed (Vmin) by a predetermined difference (VS), then controlling the transmission to trigger downshifting.

The invention relates to a method for control of an automatic transmission of a vehicle in a downhill-travel situation.

Vehicles equipped with automatic transmission, particularly hydraulically controlled transmission, have long suffered from a deficiency that becomes evident during downhill-travel situations. In fact, when the driver takes his foot off the accelerator, the vehicle adopts the longest transmission ratio or disengages every transmission ratio. As a result, the vehicle cannot benefit from the braking effect of the engine in a downhill-travel situation in order to govern the speed of the vehicle.

To remedy this shortcoming, there was proposed in European Patent 1041314 a method for control of an automatic transmission wherein the slope on which the vehicle is traveling is detected, in which case a specific rule for control of the vehicle is adopted. This rule, stored in memory in the form of a map, modifies the choice of transmission ratios in order to achieve the braking effect of the engine. It establishes a correspondence among the speed of the vehicle, the power demanded and the transmission ratio.

However, this transitional rule is adapted only to a medium downhill-travel situation. In the case of a steep slope, where the braking effect of the engine would have to be very strong, the ratio chosen is generally too long to achieve the braking effect of the engine. Similarly, in the case of a very gentle slope, the braking effect of the engine is too great and the vehicle slows down.

It is therefore one objective of the invention to provide, for control of an automatic transmission of a vehicle, a method that adapts to all downhill-travel situations in which the vehicle may find itself.

With this objective in view, the object of the invention is a method for control of an automatic transmission of a vehicle provided with an engine that drives the transmission, in which method a downhill-travel situation of the vehicle is detected and a transmission ratio is chosen such that the engine absorbs energy, characterized in that the longitudinal speed at the beginning of downhill travel is stored in memory when the vehicle begins a downhill-travel situation and, as long as the vehicle is in the downhill-travel situation, the current speed of the vehicle is compared with the speed at the beginning of downhill travel in such a way that, if the current speed exceeds the speed at the beginning of downhill travel by a predetermined deviation, the transmission is then instructed to initiate downshifting.

Thus, by virtue of the invention, the transmission ratio is chosen not according to a general rule but explicitly as a function of the vehicle situation, by initiating downshifting after analysis of this situation. In fact, the need for downshifting appears when the vehicle in the downhill-travel situation accelerates and substantially exceeds the speed at the beginning of downhill travel.

In one particular embodiment, the vehicle is equipped with a system for governing the speed.

Preferably, the downhill-travel situation is detected if the slope is greater than a predetermined threshold slope, if the power demand of the engine is smaller than a predetermined power threshold, and braking is absent. The presence of braking corresponds normally to the driver's intent to slow or even stop the vehicle, and therefore does not correspond to the situation constituting the object of the invention. The downhill-travel situation therefore corresponds to the presence of a slope on which the vehicle is traveling and to the intent of the driver to not accelerate, as indicated by a power demand below a threshold. This power demand can be evaluated by the position of the accelerator pedal, for example by the completely released position of the pedal. In the case of a vehicle equipped with a speed-governing system, the power demand is measured within the engine controller, using an equivalent of the position of the accelerator pedal. This can be the position of the butterfly valve in the case of a controlled-ignition engine, or the quantity of fuel injected in the case of a diesel engine or a setpoint of the torque to be delivered by the engine.

Preferably, the method includes an additional test step verifying that, before downshifting is initiated, the energy-absorption capacity of the engine is smaller than a predetermined power threshold. Before initiating downshifting, the method verifies that the engine is not in a situation in which it can absorb energy sufficiently. Thus downshifting is initiated only if the excess speed is due to the incapacity of the engine to hold the vehicle back. If downshifting has been initiated previously, further downshifting is avoided by allowing the vehicle time to slow and to approach the speed at the beginning of downhill travel.

In a particular manner, the absorption capacity of the engine is determined by the engine speed. In fact, it is easy to relate the power absorption capacity of the engine to its speed of revolution when the power demanded is zero (butterfly valve closed or no fuel being injected).

Advantageously, the threshold of power absorption capacity is an increasing function of the slope. The greater the slope, the higher is the predetermined power threshold that leads to downshifting as soon as the braking effect of the engine is no longer sufficient to prevent runaway acceleration of the vehicle. In the case in which the absorption capacity is determined by the engine speed, downshifting will be initiated for an engine speed threshold that increases as a function of slope. As an example, for a controlled ignition engine, the predetermined threshold is substantially 1800 to 2000 rpm for a slope of less than 5%, 2500 to 3000 rpm for a slope of 5 to 10% and 3500 to 4000 rpm for a slope of greater than 10%. These thresholds are lower in the case of a diesel engine.

As regards the deviation from predetermined speed, it is preferably between 5 and 10 km/h. The deviation from predetermined speed is in the low range on the whole in the case of a vehicle equipped with a speed governor, and otherwise is in the high range on the whole. The deviation from predetermined speed can also depend on the current transmission ratio.

The invention also has as an object a system for control of an automatic transmission of a vehicle provided with an engine that drives the transmission, the system being provided with means for identifying a downhill-travel situation of the vehicle and with means for choosing a transmission ratio so that the engine absorbs energy, characterized in that it is additionally provided with means for measuring and storing in memory the longitudinal speed at the beginning of downhill travel, measuring the said speed and storing it in memory when the vehicle begins a downhill-travel situation, means for comparing the current speed of the vehicle with the speed at the beginning of downhill travel, and means for instructing the transmission to initiate downshifting if the current speed exceeds the speed, stored in memory, at the beginning of downhill travel by a predetermined deviation.

The invention also has as an object a vehicle provided with an engine and an automatic transmission, characterized in that it is provided with the foregoing control system.

The invention will be better understood and other features and advantages will become apparent upon reading the description presented hereinafter, the description referring to the attached drawings, wherein;

FIG. 1 is a schematic view of a motive power group using a method according to the invention;

FIG. 2 is a flow chart of the method according to the invention.

A vehicle in which the method according to the invention is used is provided with a motive power group 1 composed of an engine 2 and an automatic transmission 3. Engine 2 is, for example, an internal combustion engine of the diesel or controlled ignition type that delivers power to transmission 3 then to wheels 4. Transmission 3 is, for example, an automatic gearbox with epicycloids trains, a robotic gearbox or a pulley-type speed variator. An electronic control unit 5 makes it possible to control transmission 3 either by directly determining the transmission ratio to be used or by interfering with an autonomous control system of transmission 3, such as a hydraulic system.

Electronic unit 5 utilizes the method according to the invention, for example in the form of an information-processing program. It receives information on the state of the vehicle, in particular the position F of a brake pedal 6, the position Acc of an accelerator pedal 7, the speed V of the vehicle, the engine speed NTA and information useful for determining the slope P, or possibly direct information about the slope P. The engine speed NTA is transmitted, for example, by an engine controller 8.

Referring to FIG. 2, after undergoing an initialization stage 20, the unit acquires the aforesaid data V, P, Acc, F and NTA in step 21. In test step 22, the slope P is compared with a predetermined threshold slope PS. If the slope P is greater than the threshold slope PS, it is decided that the vehicle is traveling on a downhill slope, and test step 23 is undertaken. Otherwise step 26 is undertaken.

In test step 23, the accelerator position Acc is compared with a predetermined power threshold, expressed in the form of a position threshold AccS of the accelerator. If the position Acc exceeds the threshold position AccS, step 26 is undertaken. Otherwise it is decided that the accelerator pedal is released and test step 24 is undertaken, in which the position of the brake pedal is checked. If the brakes have been activated, the position F is then equal to 1, and step 26 is undertaken. Otherwise step 25 is undertaken.

Step 25 is reached when the vehicle is in a downhill-travel situation in which the speed Vmin at the beginning of downhill travel is maintained. In contrast, in step 26, when the vehicle is not in a downhill-travel situation, the variable Vmin is updated with the current speed V of the vehicle.

At the end of one or other of these steps 25, 26, test step 27 is executed. In this step 27, the speed V is compared with the speed Vmin at the beginning of downhill travel. If the speed V is not higher than the speed Vmin at the beginning of downhill travel plus a speed deviation VS, this deviation being positive, then execution of the program is returned to step 21. This situation corresponds to a case in which the vehicle speed is not too different from the speed Vmin at the beginning of downhill travel. This is the case in particular when step 26 has just been executed, because the stated condition is always verified. In the opposite case, or in other words if the speed V is higher than the speed Vmin at the beginning of downhill travel plus a speed deviation VS, then test step 28 is executed.

During test step 28, the speed NTA of revolution of the engine is compared with a threshold speed NS. If the speed NTA is greater than the threshold speed NS, then the engine is already revolving rapidly and absorbing a power greater than the predetermined power threshold and corresponding to the speed NS of revolution. Downshifting is not authorized and execution of the program is returned to step 21. In the opposite case, step 29 is executed, whereby a downshift instruction is sent to the transmission to shorten the transmission ratio. For a transmission with continuously variable ratio, the downshift request can take the form of a request to increase the gearbox input speed. Execution of the program is then returned to step 21. As indicated hereinabove in the description of the invention, the threshold NS is a function of the slope.

Evaluation of the slope P can be achieved by a method such as disclosed, for example, in European Patent 1041314. According to this method, a calculated acceleration γc is evaluated and compared with the real acceleration γm. The evaluation of the calculated acceleration uses the formula: γc=rap(N).C _(moteur)/Rayon.Masse−½ρ.SCX.V _(veh)/Masse.g.kr,

In this expression:

-   -   rap(N) is the gear reduction ratio relative to the wheel over         the ratio (N),     -   C_(moteur) is the engine torque determined by the engine control         unit as a function, for example, of the position of the         accelerator pedal and of the engine speed,     -   Rayon is the wheel radius,     -   Masse is the empty mass of the vehicle with two persons on         board,     -   ρ is the density of the air,     -   Scx is the coefficient of penetration of the vehicle through the         air,     -   g is the acceleration of gravity (g=9.81 m/s2), and     -   kr is the resistance to rolling.

The slope P being traveled by the vehicle (positive in the case of downhill travel) is then calculated according to the following formula: P=(γm−γc)/g

The evaluation is performed by unit 5 on the basis of information received, or by another vehicle system that transmits the value of P directly to unit 5. The information received originates, for example, from sensors that are not illustrated, from engine controller 8 or from another vehicle system, possibly via an on-board digital network. European Patent 1106872 also discloses the same method. Other methods of evaluation of the slope may be used. 

1. A method for control of an automatic transmission (3) of a vehicle provided with an engine (2) that drives the transmission (3), in which method: a downhill-travel situation of the vehicle is detected and; a transmission ratio is chosen such that the engine (3) absorbs energy, characterized in that a longitudinal speed (Vmin) at the beginning of downhill travel is stored in memory when the vehicle begins a downhill-travel situation and, as long as the vehicle is in the downhill-travel situation, the current speed (V) of the vehicle is compared with the speed (Vmin) at the beginning of downhill travel in such a way that: if the current speed (V) exceeds the speed (Vmin) at the beginning of downhill travel by a predetermined deviation (VS), the transmission is then instructed to initiate downshifting.
 2. A control method according to claim 1, characterized in that the downhill-travel situation is detected if the slope (P) is greater than a predetermined threshold slope (PS), if the power demand (Acc) of the engine is smaller than a predetermined power threshold (AccS), and braking is absent.
 3. A control method according to claim 1, characterized in that it includes an additional test step (28) verifying that, before downshifting is initiated, the energy-absorption capacity of the engine is smaller than a predetermined power threshold.
 4. A control method according to claim 3, characterized in that the absorption capacity of the engine is determined by the engine speed (NTA).
 5. A control method according to claim 3, characterized in that the threshold (NS) of power absorption capacity is an increasing function of the slope (P).
 6. A control method according to claim 1, characterized in that the deviation (VS) from predetermined speed is between 5 and 10 km/h.
 7. A control method according to claim 1, characterized in that the vehicle is equipped with a speed-governing system.
 8. A system for control of an automatic transmission (3) of a vehicle provided with an engine (2) that drives the transmission (3), the system being provided with: means for identifying a downhill-travel situation of the vehicle, and means for choosing a transmission ratio so that the engine absorbs energy, characterized in that it is additionally provided with: means for measuring and storing in memory the longitudinal speed (Vmin) when the vehicle begins a downhill-travel situation, means for comparing the current speed (V) of the vehicle with the speed (Vmin) at the beginning of downhill travel, and; means for instructing the transmission to initiate downshifting if the current speed (V) exceeds the speed (Vmin) at the beginning of downhill travel by a predetermined deviation (VS).
 9. A vehicle provided with an engine and an automatic transmission, characterized in that it is provided with the system according to claim 8 for control of the automatic transmission. 